1. Field of the Invention
This invention relates to methods of encoding digital data into binary form, and decoding such binary data. The binary system, using 1's (ones) and 0's (zeroes) to represent alphanumeric data, is well known in the art. Currently, two types of binary code are used most commonly: the Baudot code, a five-hole code for representing alphanumeric characters; and the binary coded decimal (BCD) code, used to represent decimal numbers in binary form.
From the binary system's inception, the number of bits available for encoding was often less than the number of bits required to represent a letter or number. The original five-hole Baudot code, for example, permitted only 32 unique combinations, insufficient to represent the entire 26-character English alphabet in both upper case and lower case form. The BCD code, in which any decimal number can be represented in binary form, requires a great many digits to represent numbers. For example, four digits, or "bits" are needed to represent the decimal number 8.
Historically, solutions to this problem have been attempted in two categories: expanding the capacity of the encoding medium, or modifying the code interpreting system. When the size of the system that includes this coding method is inflexibly determined due to cost considerations or physical limitations, only the latter category of solutions is viable.
2. Description of the Prior Art
One of the earliest methods of binary code interpretation, still in use today, reserved unique combinations or groups of punched holes for "letter shift" and for "figure shift" control characters. The receiving, decoding machine was conditioned to interpret all hoses that followed a "letter shift" character as letters, and all holes that followed a "figure shift" character as numerals and special characters. In this way, each combination of holes served double duty; its definition was contingent on the last control character encountered.
Usage of the "letter shift" and "figure shift" control characters is typified by U.S. Pat. No. 2,619,533. An extension of this method, which uses more than two control characters, is disclosed in Pat. No. 3,530,239. Unfortunately, this class of inventions has one major drawback in practice: control characters tend to be encoded an inordinate number of times during the course of utilizing the system.
A method of using colored inks to differentiate the decimal equivalent of binary codes is shown in U.S. Pat. No. 3,486,006; Pat. No. 3,666,946 teaches the use of different photoluminescent inks to differentiate among the values of a given binary code. The disadvantage of these systems is the cost of sophisticated equipment required to discern the ink colors. U.S. Pat. No. 3,317,715 discloses using the space between columns and above and below the punched holes to store additional information on magnetic strips. This method requires the use of a magnetic read head, which is considered impractical for many applications.
The method disclosed in U.S. Pat. No. 2,836,653 determines whether uni-directional motion of a punched paper tape is present, but makes no use of this information beyond sounding an alarm when the tape ceases to move. Finally, and most recently, U.S. Pat. No. 3,968,349 makes use of the motion of the medium, but only to orient the electronic circuitry to accept data read in one direction or another. In that system, each binary code combination represents only one decimal value; the direction of medium motion information is not used to augment the existing encoded bits.